Recently, transparent glass plates are being replaced in various fields with transparent materials which are nonfragile or less fragile compared to glass materials. For example, plastic substrates, and in particular, polycarbonate resin substrates are used in various applications such as windows of buildings and automobiles and instrument covers as a replacement for glass materials in consideration of their excellent transparency, impact resistance, and heat resistance.
However, polycarbonate resin molded articles are inferior in surface properties such as scratch resistance and weatherability compared to the glass, and improvement in such surface properties is sincerely desired. Recent demands include development of polycarbonate resin molded articles which can endure more than 10 years of outdoor exposure so that the article can be used in applications such as automobile windows and highway noise barriers.
Various means have been proposed for improving the weatherability of the polycarbonate resin molded articles. Exemplary such proposals include formation of a film of acrylic resin having high weatherability on the surface of the polycarbonate resin substrate, and formation of a resin layer containing an UV absorbent on the resin surface, for example, by coextrusion.
For the improvement of scratch resistance of the polycarbonate resin molded articles, coating of a thermosetting resin such as polyorganosiloxane or melamine resin, and coating of a polyfunctional acrylic photocurable resin have been proposed.
In the meanwhile, JP-A 56-92059 and JP-A 1-149878 (Patent Documents 1 and 2) disclose a method for producing a transparent article having the weatherability simultaneously with the scratch resistance, and in a known UV absorbent transparent substrate, a protective film of a colloidal silica-containing polysiloxane is formed on a primer layer containing a large amount of UV absorbent.
However, incorporation of a large amount of UV absorbent in the primer layer generated various adverse effects such as inferior adhesion with the underlying substrate or the overlying protective coating formed by coating a colloidal silica-containing polysiloxane coating composition on the primer layer, removal of the UV absorbent from the composition by volatilization during the curing at an elevated temperature, and generation of cracks and whitening or yellowing by the bleeding out of the UV absorbent in the long term outdoor use. In the meanwhile, addition of a large amount of UV absorbent to the protective coating comprising the colloidal silica-containing polysiloxane was difficult in view of the loss of the scratch resistance.
JP-A 8-151415 (Patent Document 3) discloses use of a mixture of a benzotriazole UV absorbing vinyl monomer or a benzophenone UV absorbing vinyl monomer and a vinyl monomer which is copolymerizable with such monomer as a component of the coating composition, and formation of a protective coating of this coating composition on a synthetic resin. This protective coating, however, has only limited scratch resistance due to the use of the vinyl polymer.
JP-A 2001-114841, Japanese Patent No. 3102696, JP-A 2001-214122, and JP-A 2001-47574 (Patent Documents 4 to 7) disclose resin articles coated with a multilayer coating, and the coating has good adhesion to the resin substrate as well as high weatherability. In these resin articles, a benzotriazole UV absorbing vinyl monomer or a benzophenone UV absorbing vinyl monomer, an alkoxy silyl group-containing vinyl monomer, and a vinyl monomer which is copolymerizable with such monomers are used for the components of the coating composition.
In these articles, a coating composition containing a copolymer is used for the primer composition, and the primer coating is overcoated with a coating of a colloidal silica-containing polysiloxane resin to thereby provide scratch resistance and weatherability with the article. Despite considerable improvement in the adhesion to the overlying polysiloxane resin coating and in the weatherability, long term weatherability was still insufficient since formation of the crosslink network by the alkoxysilyl group in the primer layer was insufficient and the coating experienced deformation by the subsequent gradual crosslinking of the alkoxysilyl group or the hydroxysilyl group which was not involved in the curing, and such deformation resulted in the defects such as cracks and peeling. In addition, when the coating experienced rapid change in the ambient temperature, and in particular, in the high temperature range, the coating was more likely to experience the cracks by such post-crosslinking.
JP-A 2004-1393 (Patent Document 8) discloses an attempt to improve adhesion and crack generation of the coating by limiting difference in the coefficient of linear expansion between the substrate and the acrylic resin layer (primer layer) and the difference in the coefficient of linear expansion between the primer layer and the cured polysiloxane layer. However, long term weatherability was still insufficient because of the limited amount of the UV absorbent that could be incorporated in this primer layer.